Scientists have found signs that water may once have gurgled up through the Martian soil in hydrothermal vents similar to those in Yellowstone National Park.

The site of these proposed vents could possibly contain preserved traces of ancient Martian life, scientists say. That assumes, of course, that life might once have existed on Mars. No firm evidence for that idea has ever been found, however.

The vents evidence comes from NASA's Mars Exploration Rover Spirit. The robotic explorer found deposits of pure silica, a form of the element silicon that occurs when hot water reacts with rocks (quartz is a pure silica), in Mars' Gusev Crater in 2007. The discovery was announced briefly at the time, but scientists have now had time to fully analyze the deposits. The results are detailed in the May 23 issue of the journal Science.

Silica surprise
The silica was found when Spirit was exploring the Columbia Hills, which rise 300 feet from the middle of the flat lava plain that fills Gusev Crater. Scientists were uncertain about just what had formed these hills.

While Spirit was parked near an area known as the Tyrone site, mission scientists used the rover's Miniature Thermal Emission Spectrometer to look at some nearby "knobby outcrops," said study team member Steven Ruff of Arizona State University.

"It wasn't clear what we were seeing in the knobby outcrops because they were contaminated with dust and wind-blown soil. But I thought they might be silica-rich," Ruff said.

Surveys of other crops showed the same hints of silica, but were likewise contaminated. That's when the rover's jammed right front wheel came to the rescue. As the rover was driving in reverse, its crippled wheel dug a trench behind it.

"We aimed the Mini-TES at the trench and it showed a clear silica spectrum," Ruff said. They also analyzed the trench's white soil with the rover's Alpha Particle X-Ray Spectrometer which showed that the soil was more than 90 percent silica. "That a record high for silica on Mars," Ruff said.

Hot habitat for life?
Making such pure silica requires a lot of hot water, Ruff said. "On Earth, the only way to have this kind of silica enrichment is by hot water reacting with rocks," he said, making the connection to hydrothermal vents.

This relationship also links the silica to Home Plate, a football field-sized volcanic feature in the Columbia Hills. "Home Plate came from an explosive volcanic event with water or ice being involved," Ruff explained.

The team eventually found silica deposits in many other places nearby. Because hydrothermal vents on Earth harbor life, scientists suspect that they may once have done so on Mars. And the trace could be left in the silica deposits.

"On Earth, hydrothermal deposits teem with life and the associated silica deposits typically contain fossil remains of microbes," said study team member Jack Farmer, also of ASU.

"But we don't know if that's the case here," he added, "because the rovers don't carry instruments that can detect microscopic life."

The site stands in contrast to the one that NASA hopes will be explored by its Phoenix Mars Lander starting next week, because while Phoenix will be looking at the potential habitability of Mars' north polar region today and in the more recent geologic past, the silica deposits in Gusev Crater represent a "possible habitable environment of Mars' ancient past," Ruff told SPACE.com.